1. Field of the Invention
The present invention relates to a method of cell reselection for a mobile communication system and related mobile device, and more particularly, to a method of cell reselection for a mobile communication system and related mobile device that can initiate an emergency call immediately.
2. Description of the Prior Art
A Universal Mobile Telecommunications System (UMTS) is the 3rd generation mobile communication technology, which is generally composed of a Universal Terrestrial Radio Access Network (UTRAN) and a plurality of User Equipments (UEs). A Universal Terrestrial Radio Access Network (UTRAN) includes a Core Network, which is composed of a plurality of Node Bs and a plurality of base station controllers (or Radio Network Controllers, RNCs).
In the UMTS, a UE in idle mode establishes a Radio Resource Control (RRC) connection for signaling or data transmission. The UE sends an RRC connection request message including an RRC connection establishment cause to a Node B to request to establish an RRC connection. The Node B uses the RRC connection establishment cause to prioritize the RRC connection request from the UE, e.g. in high load situations. Therefore, the UE has to include the correct RRC connection establishment cause for the Node B to prioritize the RRC connection establishment. RRC connection establishment causes include emergency call, high priority access, Mobile Terminating (MT) access, Mobile Originating (MO) signaling and MO data.
For emergency calls, according to the specification developed by the 3rd Generation Partnership Project, whether or not a user can initiate an emergency call is related to access classes. For example, Access Class 10 is a control bit related to emergency calls; Access Classes 0 to 9 are normal access classes, and can be viewed as 10 mobile groups, where every mobile device is randomly allocated to one of the normal access classes; Access Classes 11 to 15 are special access classes, which are allocated to specific high-priority users as follows:
Access Class 11: For PLMN use;
Access Class 12: Security services;
Access Class 13: Public utilities (e.g. water/gas suppliers);
Access Class 14: Emergency services; and
Access Class 15: PLMN staff.
The access classes (i.e. Access Classes 0 to 15) are stored in the Subscriber Identity Module (SIM) card.
If a mobile device is allocated to one of the normal access classes or lacks an International Mobile Subscriber Identity (IMSI), and Access Class 10 is barred, or the mobile device is allocated to one or more access classes of Access Classes 11 to 15, such allocated access class (es) and Access Class 10 being barred, the mobile device will be restricted from initiating an emergency call. Otherwise, the mobile device may initiate an emergency call.
The network can restrict the mobile device from access due to a specific factor, including network congestion, that may invoke access class related access restriction. When the network restricts the mobile device from accessing a cell camped on by the mobile device, the cell broadcasts system information related to access class barring. The mobile device checks the system information broadcasted by the cell when attempting to access the network. When checking that the mobile device can not access the cell (e.g. initiate an emergency call), the mobile device proceeds to perform a cell reselection procedure to select a new cell. Known methods for performing the cell reselection procedure are described in detail in 3GPP TS 25.304, developed by the 3GPP, and are not further described here. Notably, in this situation, the mobile device ignores access classes checking when selecting a new cell.
After finding and camping on a new cell, the mobile device checks system information first when attempting to initiate an emergency call, then transmits an RRC connection request message to RNC of the network to request RRC connection establishment. However, if the new cell does not allow the mobile device to initiate an emergency call, that is, under Access Class being barred, the mobile device does not contain an IMSI, or one or more special access classes of Access Classes 11 to 15 are barred, the mobile device will be incapable of initiating an emergency call when checking the system information. In this situation, the mobile device will perform the cell reselection procedure again to search for another cell capable of providing emergency call service to the mobile device for the mobile device to camp on. The mobile device performs an RRC connection procedure after camping on the selected cell.
From the above, the mobile device might be incapable of initiating an emergency call after being restricted from accessing an originally camped cell by the network to select a new cell to camp on. In the case of threats to life or property, life or property might be lost if the mobile device is incapable of initiating an emergency call immediately.